1. Field of the Invention
The present invention relates to an apparatus for inputting coordinates, such as a tablet or a touch panel.
2. Description of the Related Art
A coordinate inputting apparatus detects coordinates on a plane indicated by a use's finger, a pen, or the like. The coordinate inputting apparatus is more often used in combination with other (electronic) devices than used alone. In this case, the coordinate inputting apparatus is affected by noise from the other device(s), whereby an error may occur in detecting coordinates. For example, when the detection plane of the coordinate inputting apparatus is superimposed on a screen, such as a liquid crystal display apparatus, the coordinate inputting apparatus is affected by periodic noise generated when the backlight or the frontlight of the liquid crystal display apparatus is driven, resulting in the occurrence of an error in detecting coordinates.
To avoid this, for example, Japanese Laid-Open Publication No. 10-284282 discloses the following technique. The operation of an inverter for allowing the backlight of a liquid crystal display apparatus to emit light is stopped in cycles. A position detection signal created by a touch pen is output during each stop cycle so as to prevent the position detection signal from being influenced by noise from the inverter. Moreover, the average luminance of the backlight, which is reduced by the stopping of the operation of the inverter, is compensated by adjusting the luminance of the backlight.
Japanese Laid-Open Publication No. 2000-207125 discloses another technique. The light of a liquid crystal display apparatus is turned off in synchronization with AD conversion of a detection signal by a tablet, thereby preventing the AD conversion of the detection signal from being influenced by noise from the light. The turned-off state of the light is delayed relative to a control signal for turning off the light. To cancel out such a delay, the timing of generating the control signal is moved ahead.
Although in Japanese Laid-Open Publication No. 10-284282 the position detection signal is output in synchronization with the stop cycle of the inverter operation, deviation of the response operation of the inverter, or the like, is not taken into consideration. Therefore, the influence of noise from the inverter cannot be said to be completely removed. If the stoppages in the stop cycle of the inverter operation are elongated so as to completely remove the influence of noise from the inverter, the average luminance of the backlight is largely reduced, so that the reduction in the average luminance cannot be compensated. Moreover, the control of the inverter is complicated.
In Japanese Laid-Open Publication No. 2000-207125, the light of the liquid crystal display apparatus is turned off in synchronization with the AD conversion of the detection signal from the tablet. Such synchronization control is complicated. Moreover, considering the delay of turning off the light, the timing of the control signal is determined. However, no measure is taken for the delay of the turned-on state of the light. Actually, the delay of the turn-on state of the light is greater than the delay of the turned-off state of the light. The delay of the turned-on state cannot be ignored.
In either of the above-described publications, the reduction in the average luminance of the light is compensated. However, if the reduction rate of the average luminance is great, it cannot be compensated. Nevertheless, a fundamental measure to prevent the reduction in the average luminance of the light, such as reducing the duration of the turned-off state of the light as much as possible, has not been taken.